So I've been wondering for a while just how accurate the EGT gauge on my truck is. In the winter, my truck will run around 250-275 at normal idle. During the summer, it reads around 175-200. Is the probe reading actual temps at the tip, or making a comparison between the tip and base of the probe? If its the second one, the lower temps in the summer would make sense (higher ambient temp vs winter so there is less difference, which leads to a lower reading).

Sometime it feels like I've got too much thinking time on my hands...

 

They are set at room temp from the manufacturer, so there can be a small difference there. They will also read higher in the winter and lower in the summer due to the ambient difference and it always thinking it is room temp. Shouldn't swing that wildly though, IMO. Seems like it is reading about 100* low at idle to me.

 

I think I disagree with Clay on this..

Thermocouples normally are rated for a temp area, say 100-1500* and ect. Our Thremos are sheeved, and the tip of the sensor is twisted right up in the top of that sheeve. If there is a break in the thermocouple, and the a wire is touching the sheeve somewhere other then right at the tip, it will "heat sink". In other words, the temp will change or even kind of average out a little with a cooler spot.

If your wires are twisted somewhere out side of the thermocouple, you could be heat sinking there as well.

This artical is found at E how. I suspect that your wires may be twisted somewhere outside of the thermocouple.


Where are They Used?
Thermocouples are widely used in manufacturing and industry whenever precise temperature control is required. They are rarely incorporated in the residential home, although newer HVAC units are utilizing them in a few of the more efficient models of combustion burners for the home market. Because they are so precise in reading the temperature differences down to 0.1 of a degree, the thermocouple can save energy by not having the heating or cooling activate with such a wide margin of inaccuracy.
How do Thermocouples Work?
Most thermocouples are all manufactured in the same way and operate on the same principle. They use dissimilar metals that when heated or cooled will generate a small voltage or current. The voltage is called a discreet signal. This voltage is so small most volt ohmmeters are not equipped to read the value. The voltage is in the mille volts or .001 volts. The same for current generated thermocouples they register in the mille amps or .001 amps.
Are There Ranges in Voltage?
Some models of the voltage thermocouples are fed a voltage in the range of 0 to 5 volts and feed back a higher voltage, but with a discreet change in the signal, generally again in the .001-volt range. Current generating thermocouples have a range typically, in the 4 to 20-mille-amp signal. Where the 4-mille amp is the low end and the 20-mille amp is the high side of the signal. You might also this written as 4ma to 20ma.
How are Thermocouples Made?
The thermocouples are constructed with very thin wires most in the diameter of .010 inches. They are then welded at the ends to form a circuit or conduction path. These small wires are encased in a metal tube and electrically insulated from the outer tube. Generally they are immersed in a heat conducting liquid or paste so the environment in which they are placed can be read, temperature taken and a small voltage or current generated from the difference of the two wires.
How is the Signal Interpreted?
The signal or voltage from the thermocouple is then fed into a controller which interprets the signal. Generally it amplifies the signal so it can be used to control the device that is either heating or cooling.
What Makes Thermocouple Wiring?
Many companies use proprietary substances in the thermocouple wires. So much so that if you attempt to take one apart to see the material used they will generally be ruined. Also the conducting substance can be toxic to your skin and or health. It is never recommended to take a thermocouple apart.

 

Good Artical

 

Pat, That is very good info. But the thermocouples used on our trucks are low end. They are set up to deal with vibration and the and ease of installation and, are made to sell at a competive price. I think you would see a wide range of readings if you installed 4 or 5 gauges (on the same vehicle ) made by the same manufacture. We tend to think what they say is accurate, I say it is just close enough to keep us out of trouble. If you consider the location of the thermocuple, and different ways we all install them and wire them. 100* Variation is good at best. Just my .02

 

Bob,

If what you say is true.. then I am sorry.. I am clueless on this subject. I will now remain quiet, watch and learn..

Do you have any info on these Bob?

 

Pat,
I don't have any info to back this up. I just think we are asking too much, to expect a $160.00- $200.00 gauge to be accurate in all cases. When my pyrometer gets to 1200* I start to back off. I can't afford to waste an engine on an aftermarket gauge's accuracy. My comment was out of experience with all kinds of aftermarket products. One size, won't fit all. This is just my thoughts, please don't be offended by my comments. That is not why I posted it.

 

No, no, no, no.. No ofense at all.. I promise.

I play with these things all the time, and yeah, I guess I was expecting them to run par with the units that I use. But as you said, the units I have, well, the scrap is worth 1000.00. So yeah, I need to watch and learn.. or break out my calibration tools and compare..that might be fun.

 

If you could test one with your calibration tools I would be interested to see what you find.

 

I think I am going to have to, because as i sit here, even my low end units have to pass a uniformity test, as well as be certified, so now I am really interested as well on how I can pull this out and test it. Just worried about removing it and reinstalling, yes, coward here..lol

 

From what I have gathered from Michael at Isspro, most thermocouples out there measure the difference in temp between the probe and the gauge head. So if you truck has been sitting in the sun for a while on a hot day, the inside of the truck is gonna be pretty toasty, and likely warmer than the outside temp which is why your gauge probably reads lower in the summer than the winter. I know that's how the Isspro EV works. Not sure on the EV2, but the Performax is set up different (can't remember the details now) to give a more accurate display of what the EGT is.

 

I would be interested to see the results as well.

 

Here is some info from Michael on it, and it explains a little on the differences between gauges as well. This info also makes sense why I never see mine change from summer to winter, and always idle at the same temp.

One significant benefit of ISSPRO Performax™ and EV² gauge systems is the temperature compensated pyrometer, unique in automotive pyrometers.

Conventional pyrometers just measure the difference in temperature between the probe tip and the leadwire end (typically at the gauge head). The industry standard is to calibrate on the assumption that the gauge head is at 70° F. In an enclosed cab on a hot day with a dark colored interior, a gauge can reach well over 170° F and will be very slow to cool off (the pods act as a good insulator). This will result in your pyro displaying a temperature over 100° F cooler than the actual temperature, in conditions where you are likely to see high temperatures. Because of this inherent inaccuracy, ISSPRO Performax and EV² pyrometers utilize temperature compensation circuits and show the actual probe tip temperature.

 

But what I dont understand is you have the box that is wired from the thermocouple, then and output to the gage. You then have your power wires. This ( in my mind ) makes this an amplifier of sort.
In other words, for a quick reference, 1ma = 1vdc output, making your gage nothing more then a volt meter.

What you are saying is that the box is actually a reference place, and not an amplifeir??

 

Thats why Tractor trucks read post temps after turbine, and run a 215 stat on coolant temp, reading it before it could read high and low depending on how far you are from the cylinder head!